Signal transmission method using MBSFN subframe in radio communication system

ABSTRACT

A signal transmission method using a multicast broadcast single frequency network (MBSFN) subframe in a radio communication system is provided. The method includes: selecting a plurality of relay stations for transmitting a plurality of backhaul signals, and transmitting the plurality of backhaul signals to the selected respective relay stations by using different radio resources, wherein each of the selected relay stations configures a subframe for receiving each of the plurality of backhaul signals as the MBSFN subframe. Accordingly, the backhaul signal can be transmitted between the base station and the plurality of relay stations by effectively using the radio resources.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Pat. application Ser. No.13/143,331, filed Jul. 5, 2011, now U.S. Pat. No. 9,413,450, which isthe National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2010/000186, filed on Jan. 12, 2010, which claimsthe benefit of earlier filing date and right of priority to KoreanApplication No. 10-2009-0048045, filed on Jun. 1, 2009, and also claimsthe benefit of U.S. Provisional Application Nos. 61/250,835, filed onOct. 12, 2009, 61/144,744, filed on Jan. 15, 2009, and 61/144,460, filedon Jan. 14, 2009, the contents of which are all incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a signal transmission method using amulticast broadcast single frequency network (MBSFN) subframe in a radiocommunication system. More particularly, the present invention relatesto a method of transmitting a backhaul signal between a base station anda relay station by using an MBSFN subframe and a method of reporting ausage of the MBSFN subframe to a user equipment.

BACKGROUND ART

Radio communication systems are widely spread all over the world toprovide various types of communication services such as voice or data.In general, the radio communication system may be a multiple accesssystem capable of supporting communication with multiple users bysharing available radio resources. Examples of the radio resourceinclude a time resource, a frequency resource, a space resource, etc.Examples of the multiple access system include a time division multipleaccess (TDMA) system, a frequency division multiple access (FDMA)system, a spatial division multiple access (SDMA) system, etc. Time,frequency, and space resources are primary radio resources differentlyallocated to multiple users respectively to the TDMA system, the FDMAsystem, and the SDMA system.

In addition, the radio communication system is a system supportingbidirectional communication. The bidirectional communication can beperformed by using a time division duplex (TDD) mode, a frequencydivision duplex (FDD) mode, etc. The TDD mode uses the time resource toidentify uplink transmission and downlink transmission. The FDD modeuses the frequency resource to identify uplink transmission and downlinktransmission.

The radio communication system includes a base station (BS) providing aservice to a specific region (i.e., a cell). In general, a userequipment (UE) can communicate with the BS when the UE is located withina coverage of the BS. When the UE is located in a cell boundary or whenan obstacle such as a building exists between the UE and the BS,communication quality between the UE and the BS may not be satisfactory.

Several methods are provided to extend the coverage of the BS. In one ofthe methods, the radio communication system employs a relay station(RS). For example, long term evolution (LTE)-advanced, which is apromising candidate technique of international mobile telecommunication(IMT)-advanced (i.e., a post 3^(rd) generation mobile communicationsystem), includes an RS technique among primary techniques.

The RS is an apparatus for relaying a signal between the BS and the UE,and is used to extend cell coverage of the radio communication systemand to improve cell throughput. An uplink and a downlink between the BSand the RS are backhaul links. An uplink and a downlink between the BSand the UE or between the RS and the UE are access links. Hereinafter, asignal transmitted through the backhaul link is referred to as abackhaul signal, and a signal transmitted through the access link isreferred to as an access signal.

It is difficult for the RS to transmit and receive a signal by using thesame frequency band and the same time. For example, it is difficult forthe RS to transmit an access signal while receiving a backhaul signal.This is because the access signal transmitted by the RS and the backhaulsignal received by the RS act as interference to each other, which mayresult in signal distortion. This is called self interference (SI). Inorder for the RS to solve the SI problem, a complex SI cancellationprocess is required, and transmission and reception signal processorsneed to be separated spatially. In reality, it is difficult for the RSto cancel the SI, and even if it is implemented, great expenses arerequired.

The RS needs to report to UEs connected to the RS a subframe in whichthe backhaul signal is received from the BS. This is to prevent the UEsfrom performing an unnecessary signal reception operation since the RScannot transmit the access signal in the subframe due to the SI. As onemethod of reporting the subframe by the RS to the UE, there is a methodof configuring a subframe for receiving a backhaul signal as a multicastbroadcast single frequency network (MBSFN) subframe.

The RS may report to the UE that a corresponding subframe is an MBSFNsubframe by using a control signal transmitted in a duration of a firstspecific OFDM symbol and then may receive a backhaul signal in aduration of the remaining OFDM symbols. Such a method can be referred toas a relay method based on the MBSFN subframe.

As described above, although the MBSFN subframe can be used forreceiving the backhaul signal by the RS from the BS, the MBSFN subframeis used in principle for a multimedia broadcast multicast service(MBMS). However, a method of identifying a usage of the MBSFN subframeand reporting the usage to the UE has not been considered in theconventional technique.

In addition, when one BS transmits a backhaul signal to two or more RSsin the relay method based on the MBSFN subframe, a multiplexing methodfor improving usage efficiency of radio resources is not taken intoaccount.

SUMMARY OF INVENTION Technical Problem

The present invention provides a signal transmission method using amulticast broadcast single frequency network (MBSFN) subframe.

Technical Solution

According to an aspect of the present invention, a method oftransmitting a backhaul signal in a radio communication system isprovided. The method includes: selecting a plurality of relay stationsfor transmitting a plurality of backhaul signals, and transmitting theplurality of backhaul signals to the selected respective relay stationsby using different radio resources, wherein each of the selected relaystations configures a subframe for receiving each of the plurality ofbackhaul signals as a multicast broadcast single frequency network(MBSFN) subframe.

Advantageous Effects

According to the present invention, a backhaul signal can be transmittedbetween a base station and a plurality of relay stations by effectivelyusing radio resources. In addition, a user equipment can know a usage ofa multicast broadcast single frequency network (MBSFN) subframe, andthus can perform only a necessary decoding operation based on the usage.Therefore, unnecessary power consumed in the user equipment can bereduced.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a radio communication system.

FIG. 2 shows a radio communication system employing a relay station.

FIG. 3 shows a structure of a frequency division duplex (FDD) radioframe in 3^(rd) generation partnership project (3GPP) long termevolution (LTE).

FIG. 4 shows a time division duplex (TDD) radio frame structure in 3GPPLTE.

FIG. 5 shows an example of a resource grid for one downlink slot.

FIG. 6 shows an example of a resource grid for one uplink slot.

FIG. 7 shows an example of a structure of a multicast broadcast singlefrequency network (MBSFN) subframe.

FIG. 8 is a flowchart showing a method of transmitting a backhaul signalaccording to an embodiment of the present invention.

FIG. 9 shows a case where a subframe offset value of a relay station is1.

FIG. 10 shows frequency band allocation in each subframe when using afrequency division multiplexing (FDM) scheme.

FIG. 11 shows an example in which a backhaul signal can be transmittedonly for some relay stations among a plurality of relay stations.

FIG. 12 shows an example in which relay stations 1 to 3 which aregrouped by a base station have the same subframe offset value.

FIG. 13 shows resource allocation of each subframe when using a spatialdivision multiplexing (SDM) scheme.

FIG. 14 shows radio resource allocation when a backhaul signal istransmitted by using a time division multiplexing (TDM) scheme.

FIG. 15 shows an example in which a radio resource is wasted when abackhaul signal is transmitted to a plurality of relay stations by usingan FDM scheme.

FIG. 16 shows a radio communication system for performing cooperativeretransmission among a plurality of relay stations.

FIG. 17 shows a relay station for establishing a backhaul link with adifferent base station in a different sector.

FIG. 18 is a block diagram showing a base station according to anembodiment of the present invention.

MODE FOR INVENTION

Wideband CDMA (WCDMA) can be implemented with a radio technique such asa universal terrestrial radio access network (UTRAN) defined by the3^(rd) generation partnership project (3GPP) standardizationorganization. CDMA2000 is a radio technique based on code divisionmultiple access (CDMA). High rate packet data (HRPD) defined by the3^(rd) generation partnership project 2 (3GPP2) provides a high-ratepacket data service in a CDMA2000-based system. Evolved HRPD is anevolution of the HRPD. Time division multiple access (TDMA) can beimplemented with a wireless technique such as global system for mobilecommunications (GSM)/general packet radio service (GPRS)/enhanced datarates for GSM evolution (EDGE). Orthogonal frequency division multipleaccess (OFDMA) can be implemented with a wireless technique such as IEEE802.11(Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, evolved-UTRAN (E-UTRAN),etc.

Long term evolution (LTE) is a part of an evolved-universal mobiletelecommunications system (E-UMTS) using an evolved-universalterrestrial radio access network (E-UTRAN). The LTE employs the OFDMA ina downlink and employs single carrier-frequency division multiplexaccess (SC-FDMA) in an uplink. LTE-advanced (LTE-A) is an evolution ofthe LTE. For clarity, the following description will focus on the 3GPPLTE/LTE-A. However, technical features of the present invention are notlimited thereto.

FIG. 1 shows a radio communication system.

Referring to FIG. 1, a radio communication system 10 includes at leastone base station (BS) 11. The BSs 11 provide communication services tospecific geographical regions (generally referred to as cells) 15 a, 15b, and 15 c. The cell can be divided into a plurality of regions(referred to as sectors). One BS may include one or more cells. The BS11 is generally a fixed station that communicates with a user equipment(UE) 12 and may be referred to as another terminology, such as anevolved node-B (eNB), a base transceiver system (BTS), an access point,an access network (AN), etc.

The UE 12 may be fixed or mobile, and may be referred to as anotherterminology, such as a mobile station (MS), a user terminal (UT), asubscriber station (SS), a wireless device, a personal digital assistant(PDA), a wireless modem, a handheld device, an access terminal (AT),etc.

Hereinafter, a downlink (DL) denotes a communication link from the BS tothe UE, and an uplink (UL) denotes a communication link from the UE tothe BS. In the DL, a transmitter may be a part of the BS, and a receivermay be a part of the UE. In the UL, the transmitter may be a part of theUE, and the receiver may be a part of the BS. In UL transmission, asource station may be the UE, and a destination station may be the BS.In DL transmission, the source station may be the BS, and thedestination station may be the UE.

The radio communication system may include a relay station (RS). The RSmay be a UE, or may be provided as a separate RS. The BS may performfunctions such as connectivity, management, control, and resourceallocation between the RS and the UE. The RS can be also referred to asother terms such as a relay node (RN).

FIG. 2 shows a radio communication system employing an RS.

Referring to FIG. 2, a BS 20 communicates with a UE 30 via an RS 25. InUL transmission, the UE 30 transmits UL data to the BS 20 and the RS 25,and the RS 25 retransmits the received UL data. In DL transmission, theBS 20 also communicates with a UE 31 via RSs 26 and 27. In DLtransmission, the BS 20 transmits DL data to the RSs 26 and 27 and theUE 31, and the RSs 26 and 27 retransmit the received DL data eithersimultaneously or in sequence. Although it is shown that two RSs 26 and27 are used in DL transmission, the present invention is not limitedthereto, and thus one RS may be used. In addition, although one BS 20,three RSs 25, 26, and 27, and two UEs 30 and 31 are shown in FIG. 2, thepresent invention is not limited thereto. The number of BSs, RSs, andUEs included in the radio communication system is not limited to anyparticular number. A relay scheme used in the RS may be either amplifyand forward (AF) or decode and forward (DF), and the technical featuresof the present invention are not limited thereto.

Even UEs located within the coverage of the BS can communicate with theBS via the RS in order to improve a transfer speed depending on adiversity effect. Hereinafter, a macro user equipment (MaUE) is a UEthat directly communicates with the BS, and a relay user equipment(ReUE) is a UE that communicates with the RS. The MaUE and the ReUE willbe collectively referred to as the UE unless otherwise specified.

FIG. 3 shows a structure of a frequency division duplex (FDD) radioframe in 3GPP LTE.

Referring to FIG. 3, a radio frame includes 10 subframes. One subframeincludes two consecutive slots. A time required for transmitting onesubframe is called a transmission time interval (TTI). For example, onesubframe may have a length of 1 millisecond (ms), and one slot may havea length of 0.5 ms.

FIG. 4 shows a time division duplex (TDD) radio frame structure in 3GPPLTE.

Referring to FIG. 4, one radio frame has a length of 10 ms, and consistsof two half-frames each having a length of 5 ms. One half-frame consistsof five subframes each having a length of 1 ms. Each subframe isdesignated as any one of a UL subframe, a DL subframe, and a specialsubframe. One radio frame includes at least one UL subframe and at leastone DL subframe. One subframe consists of two consecutive slots. Forexample, one subframe may have a length of 1 ms, and one slot may have alength of 0.5 ms.

The special subframe is a specific period positioned between the ULsubframe and the DL subframe for the purpose of UL-DL separation. Oneradio frame includes at least one special subframe. The special subframeincludes a downlink pilot time slot (DwPTS), a guard period, and anuplink pilot time slot (UpPTS). The DwPTS is used for initial cellsearch, synchronization, or channel estimation. The UpPTS is used forchannel estimation in a BS and UL transmission synchronization of a UE.The guard period is positioned between the UL time slot and the DL timeslot and is used to remove interference that occurs in UL transmissiondue to a multi-path delay of a DL signal.

In FDD and TDD radio frames, one slot includes a plurality of orthogonalfrequency division multiplexing (OFDM) symbols in a time domain and aplurality of resource blocks (RBs) in a frequency domain. Since 3GPP LTEuses OFDMA in DL transmission, the OFDM symbol is for expressing onesymbol period, and thus can be referred to as other terms such as anSC-FDMA symbol according to the multiple access scheme. The RB is aresource allocation unit, and includes a plurality of consecutivesubcarriers in one slot.

The sections 4.1 and 4.2 of 3GPP TS 36.211 V8.3.0 (2008-05) “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation (Release 8)” canbe incorporated herein by reference in order to explain the radio framestructure described with reference to FIG. 3 and FIG. 4.

The structure of the radio frame is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe, and the number of OFDM symbolsincluded in the slot may change variously.

FIG. 5 shows an example of a resource grid for one DL slot.

Referring to FIG. 5, one DL slot includes a plurality of OFDM symbols ina time domain. It is described herein that one DL slot includes 7 OFDMAsymbols and one resource block (RB) includes 12 subcarriers in afrequency domain for exemplary purposes only, and the present inventionis not limited thereto.

Each element on the resource grid is referred to as a resource element,and one RB includes 12×7 resource elements. The number N^(DL) of RBsincluded in the DL slot depends on a DL transmission bandwidthdetermined in a cell.

FIG. 6 shows an example of a resource grid for one UL slot.

Referring to FIG. 6, the UL slot includes a plurality of SC-FDMA symbolsin a time domain, and includes a plurality of resource blocks (RBs) in afrequency domain. Although it is described herein that one UL slotincludes 7 SC-FDMA symbols, and one resource block includes 12subcarriers, the present invention is not limited thereto. The numberN^(UL) of resource blocks included in the UL slot depends on a ULtransmission bandwidth defined in a cell.

An RS can configure a subframe for receiving a backhaul signal from a BSas an MBSFN subframe.

First, the MBSFN subframe will be described. The MBSFN subframe can beused for two usages. The first usage is for a multimedia broadcastmulticast service (MBMS). The MBMS is a service for transmitting thesame signal concurrently in several cells in a radio communicationsystem. Therefore, a reference signal has to be inserted in a differentmanner from that of unicast transmission in which different data istransmitted in each cell. For this, a BS reports to a UE a position of asubframe in which an MBMS signal is transmitted, and a reference signalis inserted in a different manner from that of unicast transmission inthe subframe. The UE can receive the MBMS signal in the MBSFN subframefor this usage. For convenience of explanation, the MBSFN subframe usedfor this usage is hereinafter referred to as a true (T)-MBSFN subframe.

The second usage is for preventing a UE connected to each of a BS or anRS from performing unnecessary signal reception operations and referencesignal measurement. For example, there is a possibility ofmalfunctioning if the UE fails to receive any signal including areference signal in the entire part of a specific subframe in 3GPP LTE.To prevent this, the RS configures a subframe for receiving a backhaulsignal from the BS as an MBSFN subframe, and reports this to an ReUE.Then, the ReUE does not perform reference signal measurement in theMBSFN subframe for this usage. Although the BS or the RS configures thissubframe as the MBSFN subframe, the subframe in which no MBMS signal istransmitted in practice is hereinafter referred to as a fake (F)-MBSFNsubframe for convenience of explanation.

A method in which the BS or the RS identifies a usage of the MBSFNsubframe and reports the usage to the UE has not been taken into accountin the conventional technique.

FIG. 7 shows an example of a structure of an MBSFN subframe.

Referring to FIG. 7, the MBSFN subframe may include a control region, aguard period 1, a guard period 2, and a data region.

The control region is a region to which control channels are allocatedin a specific number of OFDM symbol periods (e.g., 2 OFDM symbolperiods). Examples of DL control channels used in 3GPP LTE include aphysical control format indicator channel (PCFICH), a physicalhybrid-ARQ indicator channel (PHICH), a physical downlink controlchannel (PDCCH), etc. The PCFICH carries information regarding thenumber of OFDM symbols (i.e., a size of the control region) used fortransmission of control channels in the subframe. The PHICH carries anacknowledgement (ACK)/not-acknowledgement (NACK) signal for UL hybridautomatic repeat request (HARQ). That is, an ACK/NACK signal for UL datatransmitted by the UE is transmitted on the PHICH.

Control information transmitted on the PDCCH is referred to as downlinkcontrol information (DCI). The DCI indicates UL resource allocationinformation, DL resource allocation information, and UL transmit powercontrol commands for any UE group, etc. The PDCCH can carry a downlinkshared channel (DL-SCH)'s resource allocation and transmission format,an uplink shared channel (UL-SCH)'s resource allocation information,paging information regarding a paging channel (PCH), system informationregarding the DL-SCH, a resource allocation of a higher-layer controlmessage such as a random access response transmitted on the PDSCH, anaggregation of transmit power control commands for individual UEs in anyUE group, activation of a voice over Internet (VoIP), etc.

Each of the guard period 1 and the guard period 2 may include, forexample, one OFDM symbol period, and is a period for cancellinginterference between data transmission and data reception. The guardperiod 1 and the guard period 2 may change variously depending on apropagation delay between a BS and an RS.

The data region is located between the guard period 1 and the guardperiod 2. A physical downlink shared channel (PDSCH) can be allocated tothe data region.

The BS or the RS can first report to a UE whether any subframe is theMBSFN subframe by using a higher layer signal. For example, the BS orthe RS can transmit information indicating whether the subframe is theMBSFN subframe through a physical broadcast channel (PBCH) on whichsystem information is transmitted. The PBCH can be transmitted in firstfour OFDM symbols of a 2^(nd) slot of a 1^(st) subframe. The informationindicating whether the subframe is the MBSFN subframe may have a bitmapformat. For example, if a specific subframe is the MBSFN subframe, theinformation may be set to 1, and if the specific subframe is not theMBSFN subframe, the information may be set to 0 (the other way around isalso possible). By using the information indicating whether the subframeis the MBSFN subframe, the UE can know whether each subframe included ina radio frame is the MBSFN subframe or a normal subframe (i.e., asubframe other than the MBSFN subframe).

Regarding the subframe which is configured as the MBSFN subframe, the BSor the RS can report its usage by using the higher layer signal or thePDCCH. That is, if information indicating the usage of the MBSFNsubframe is referred to as a usage indicator for convenience ofexplanation, the usage indicator can be transmitted by using the higherlayer signal or the PDCCH.

First, a case where the usage indicator is transmitted by using thehigher layer signal will be described. The BS or the RS can transmit theusage indicator by appending the usage indicator to the higher layersignal such as system information transmitted through the PDSCH (e.g.,PBCH). In this case, the usage indicator may have a bitmap format.

For example, it is assumed that the BS or the RS configures a subframerepetitively with a period of P subframes, and M subframes areconfigured as MBSFN subframes in one period. In this case, the MBSFNsubframes can be expressed by a subframe kP+n₁, a subframe kP+n₂, . . ., and a subframe kP+n_(M) (herein, k is an integer, and n₁<n₂< . . .<n_(M), where n₁ to n_(M) are integers). Then, the BS or the RS cantransmit an M-bit bitmap (i.e., usage indicator) for indicating a usageof each of corresponding M subframes by using the higher layer signal.In the M-bit bitmap, “1” may indicate a T-MBSFN subframe, and ‘0’ mayindicate an F-MBSFN subframe (the other way around is also possible).

By receiving the usage indicator indicating such a usage, the UE canknow whether each MBSFN subframe is the T-MBSFN subframe or the F-MBSFNsubframe. For example, if the usage indicator received by the UE is‘0010000100’, it can be seen that a 3^(rd) MBSFN subframe and an 8^(th)MBSFN subframe among 10 MBSFN subframes are T-MBSFN subframes. Then, theUE may receive an MBMS signal in the 3^(rd) and 8^(th) MBSFN subframes,and may not perform reference signal measurement in the remaining MBSFNsubframes.

Unlike this, a backhaul signal (to be finally delivered to the UEitself) can be received by the UE in advance in the F-MBSFN subframe inwhich a signal is transmitted from the BS to the RS, and thus can beutilized to decode the backhaul signal to be relayed from the RS to theUE.

If the UE is unable to receive or decode the usage indicator, all MBSFNsubframes can be regarded as T-MBSFN subframes (or F-MBSFN subframes).

Hereinafter, a case where the usage indicator is transmitted through thePDCCH will be described.

The case where the usage indicator is transmitted through the PDCCH canbe classified into two cases: 1) a case where it is transmitted througha PDCCH of the MBSFN subframe; and 2) a case where it is transmittedthrough a PDCCH of a subframe other than the MBSFN subframe.

First, the case 1) will be described. As described above, in the MBSFNsubframe, the PDCCH can be transmitted on a specific number of OFDMsymbols located in a first portion. A BS or an RS can transmit the PDCCHby appending the usage indicator to the PDCCH of the MBSFN subframe. Inother words, by generating a new field indicating a usage of the MBSFNsubframe in the PDCCH of the MBSFN subframe and by transmitting a valueof the usage indicator in the new field, the usage of the MBSFN subframecan be reported to the UE. In this case, for example, if the usageindicator value is 1, it may indicate the T-MBSFN subframe, and if theusage indicator value is 0, it may indicate the F-MBSFN subframe. On thecontrary, if the usage indicator value is 1, it may indicate the F-MBSFNsubframe, and if the usage indicator value is 0, it may indicate theT-MBSFN subframe. When the UE is unable to receive the usage indicatoror when the UE is unable to decode the usage indicator even if the usageindicator is received, a corresponding MBSFN subframe can be regarded aseither the T-MBSFN subframe or the F-MBSFN subframe by definition.

As described above, in addition to a method in which the usage indicatorof the MBSFN subframe is directly included in the PDCCH, a usage of theMBSFN subframe can be reported by masking a specific broadcastidentifier to the PDCCH of the MBSFN subframe.

In general, the BS appends a cyclic redundancy check (CRC) for errordetection to a PDCCH (specifically, DCI) to be sent to the UE. Anidentifier (referred to as a radio network temporary identifier (RNTI))is masked to the CRC according to an owner or usage of the PDCCH.Examples of the identifier include a cell-RNTI (C-RNTI) which is aunique identifier of a specific UE, a paging-RNTI (P-RNTI) which is apaging identifier for a paging message transmitted through a pagingchannel (PCH), a system information-RNTI (SI-RNTI) which is a systeminformation identifier for system information transmitted through aDL-SCH, etc.

The BS or the RS can perform transmission by masking a specificbroadcast identifier such as a specific P-RNTI and a specific SI-RNTIamong the aforementioned identifiers to only a PDCCH of the F-MBSFNsubframe among MBSFN subframes(alternatively, only a PDCCH of theT-MBSFN subframe). Then, the UE can know a usage of the MBSFN subframeaccording to whether the PDCCH having the specific broadcast identifieris detected from the MBSFN subframe. That is, when the PDCCH having thespecific identifier is detected from the MBSFN subframe, the UE canregard the MBSFN subframe as the F-MBSFN subframe. It is enough for theUE to perform an operation corresponding to the usage of the MBSFNsubframe. For example, the UE may receive the MBMS signal in the T-MBSFNsubframe, and may not measure a reference signal in the F-MBSFNsubframe. Unlike this, in a case where the UE receives a backhaul signaland utilizes the backhauls signal to decode a signal transmitted by theRS, the UE may receive the backhaul signal in the F-MBSFN subframe.

The BS or the RS can perform transmission by using a broadcastidentifier (e.g., SI-RNTI, P-RNTI) in the PDCCH of the MBSFN subframeincluding the usage indicator so that it can be received by all UEs.

Now, the case 2) where the usage indicator is transmitted through aPDCCH of a subframe other than an MBSFN subframe will be described.

The BS or the RS can transmit the usage indicator through the PDCCH ofthe subframe other than the MBSFN subframe. In this case, in order toallow all UEs to be able to receive the usage indicator, the usageindicator can be transmitted through a PDCCH transmitted in a subframethat cannot be configured as the MBSFN subframe (e.g., any one ofsubframes 0, 4, 5, and 9 in FDD, and any one of subframes 0, 1, 5, and 6in TDD).

The usage indicator transmitted in one subframe can report a usage ofone subsequent MBSFN subframe, and also can report a usage of two ormore subsequent MBSFN subframes. If the number of subsequent MBSFNsubframes is N, the usage indicator may have a bitmap format consistingof N bits. For example, if a 3-bit usage indicator (e.g., ‘101’) istransmitted on a PDCCH of a subframe 0, each bit value of the usageindicator may indicate a usage of three MBSFN subframes subsequent tothe subframe 0. If subframes 1, 2, and 3 are MBSFN subframes, thesubframe 1 and 3 may indicate a T-MBSFN subframe, and the subframe 2 mayindicate an F-MBSFN subframe. Alternatively, if the subframes 1 and 3are MBSFN subframes, a 2-bit usage indicator can be transmitted.

If the UE is unable to receive the usage indicator, all MBSFN subframescan be regarded as T-MBSFN subframes or F-MBSFN subframes until a nextusage indicator is received. Alternatively, the UE may operate under theassumption that the same content as the previously received usageindicator is received.

As described above, the usage indicator can be transmitted to the UE byusing the higher layer signal or the PDCCH. The usage indicator mayadditionally include a detailed usage indicator which is information forreporting a detailed usage of the F-MBSFN subframe.

The detailed usage of the F-MBSFN subframe may be various, for example:a) a usage of receiving a backhaul signal by the RS from the BS; b) ausage of transmitting a positioning reference signal by the BS or theRS; c) a usage of transmitting a unicast signal depending on 3GPPrelease 9; and d) a usage of transmitting a unicast signal depending on3GPP release 10, etc. The detailed usage indicator refers to informationfor reporting the detailed usage of the F-MBSFN subframe.

One of reasons of specifying the usage of the F-MBSFN subframe in afurther detail as described above is that a reference signal transmittedin the F-MBSFN subframe depending on each detailed usage can vary. Thatis, in case of an F-MBSFN subframe used for receiving a backhaul signalby the RS from the BS, any signal including the reference signal may notbe transmitted in a region other than a PDCCH region of the F-MBSFNsubframe. In case of an F-MBSFN subframe used for transmitting apositioning reference signal by the BS or the RS, the positioningreference signal may be transmitted together with a cell-specificreference signal. In case of an F-MBSFN subframe used for transmitting aunicast signal based on 3GPP release 9, a cell-specific reference signalis transmitted also in a region other than a region for transmitting aPDCCH. In case of an F-MBSFN subframe used for transmitting a unicastsignal based on 3GPP release 10, the cell-specific reference signal istransmitted only in the region for transmitting the PDCCH. That is, adetailed usage of the F-MBSFN subframe and a reference signal dependingon the detailed usage are related to each other.

Therefore, the BS or the RS can perform transmission by appending adetailed usage indicator, which reports arrangement of the detailedusage of the F-MBSFN subframe and/or arrangement of the referencesignal, to the usage indicator. The detailed usage indicator can also begiven in a bitmap format. For example, if the detailed usage of theF-MBSFN subframe includes four cases as described above, i.e., the casesa) to d), then the bitmap can be configured such that 2 bits indicateone detailed usage. That is, ‘00’ may indicate a usage of the case a),‘01’ may indicate a usage of the case b), ‘10’ may indicate a usage ofthe case c), and ‘11’ may indicate a usage of the case d). If there are2^(K) types of detailed usages of the F-MBSFN subframe, the bitmap ofthe detailed usage indicator can be configured in such a manner that Kbits indicate one detailed usage. The bitmap of the detailed usageindicator may be transmitted in combination with the bitmap of the usageindicator, or may be transmitted independently.

The UE can know whether an MBSFN subframe is an F-MBSFN subframe byusing the usage indicator. When the UE knows that the MBSFN subframe isthe F-MBSFN subframe, the UE can determine a detailed usage of theF-MBSFN subframe by using the detailed usage indicator and can decode areference signal according to the determined detailed usage.

FIG. 8 is a flowchart showing a method of transmitting a backhaul signalaccording to an embodiment of the present invention.

Referring to FIG. 8, a BS selects some RSs for transmitting the backhaulsignal among a plurality of RSs (step S100). For example, the BS mayselect an RS 1 and an RS 2 among the plurality of RSs. The BS exchangesconfiguration signals for backhaul signal transmission with the selectedRS 1 and RS 2 (step S200 and step S210). The configuration signal mayinclude information on a radio resource allocated for backhaul signaltransmission and a usage indicator indicating a usage of a subframe inwhich the backhaul signal is transmitted (herein, a detailed usageindicator may also be included). The information on the radio resourcemay include a variety of information such as a frequency band at whichthe backhaul signal is transmitted, a position of a subframe allocatedfor backhaul signal transmission within each frequency band, a subframeoffset value, a code, etc. The subframe offset value will be describedbelow. The usage indicator may be transmitted through a PDCCH asdescribed above, or may be transmitted by using a higher layer signalnot only to an RS but also to a MaUE and an ReUE. The usage indicatormay indicate an F-MBSFN subframe, and the detailed usage indicator mayindicate, for example, the usage of case a) above.

The RS 1 and the RS 2 configure a subframe in which the backhaul signalis received as an MBSFN subframe (step S300 and step S310).

The BS transmits the backhaul signal to each of the two or more selectedRSs by allocating different radio resources. Herein, the radio resourcemay be any one of, for example, a frequency resource, a time resource,an antenna resource (for utilizing a different spatial resource), and acode resource. For example, the BS transmits the backhaul signal byusing different frequency bands f1 and f2 to the RS 1 and the RS 2 (stepS400 and step S410).

FIG. 9 shows a case where a subframe offset value of an RS is 1.

Referring to FIG. 9, the subframe offset value denotes a start point ofa radio frame. The subframe offset value implies a point at which asubframe 0 810 of a radio frame of the RS is located with respect to aposition of a subframe 0 820 of a radio frame of a BS. The subframe 0810 of the RS is spaced apart by one subframe from the subframe 0 820 ofthe BS and thus is located at a point where the subframe 1 830 of the BSis located. Therefore, the subframe offset value of the RS can be givento 1.

Hereinafter, a process of transmitting a backhaul signal by a BS to eachof a plurality of RSs by using different radio resources will bedescribed in detail. First, a case where the backhaul signal istransmitted by the BS to a plurality of selected RSs by using differentfrequency bands, that is, a frequency division multiplexing (FDM)scheme, will be described. The BS can transmit the backhaul signal totwo or more RSs by allocating different frequencies within the samesubframe.

FIG. 10 shows frequency band allocation in each subframe when using anFDM scheme.

Referring to FIG. 10, a BS transmits a backhaul signal to an RS 1, an RS2, and an RS 3 in a subframe n. In this case, a frequency band 1 may beallocated to the RS 1, a frequency band 2 may be allocated to an RS 2,and a frequency band 3 may be allocated to an RS 3. In addition, the BStransmits a backhaul signal to the RS 2, the RS 3, and an RS 4 in asubframe m. In this case, the frequency band 1 may be allocated to theRS 2, the frequency band 3 may be allocated to the RS 3, and thefrequency band 2 may be allocated to the RS 4. That is, the BS transmitsthe backhaul signal by allocating different frequency bands torespective subframes with respect to some or all of a plurality of RSsconnected to the BS. In this case, the BS may transmit the backhaulsignal to different RSs in each frame, and an allocated frequency bandmay also change variously.

The aforementioned FDM scheme has an advantage in that frequencyselectivity can be well utilized. Since a channel state of a channelbetween the BS and the RS changes slowly in general, a backhaul signalis preferably transmitted by allocating a frequency band having a goodchannel state to each RS. The BS can allocate the frequency band havinga good channel state to each RS by using channel quality informationwhich is fed back by each RS. If the RS is unable to feed back thechannel state information or if it is ineffective to feed back thechannel state information, then the BS can disperse the frequency bandallocated to each RS to the maximum extent possible. In this case,frequency diversity can be improved.

When the BS transmits the backhaul signal to the plurality of RSs byusing the FDM, if RSs are arbitrarily selected, there may be a casewhere the backhaul signal cannot be received from some or all of theselected RSs.

FIG. 11 shows an example in which a backhaul signal can be transmittedonly for some RSs among a plurality of RSs.

Referring to FIG. 11, an RS cannot receive a signal from a BS insubframes 0, 4, 5, and 9 (i.e., subframes indicated by a hatched box inFIG. 11), for example, in a 3GPP LTE system. This is because the RS hasto transmit to a UE an essential signal such as a primarysynchronization signal (PSS), a secondary synchronization signal (SSS),and a paging message. If each RS has a different subframe offset valueas shown in FIG. 11, there may be a case where at least one subframe ofeach RS corresponds to the subframes 0, 4, 5, and 9 and thus the FDMcannot be used.

In order to solve such a problem, if the FDM scheme is used, in thebackhaul signal transmission method according to the present embodiment,the step of selecting a plurality of RSs by the BS may further include astep of dividing RSs in a cell into a plurality of groups and a step ofselecting a group including some RSs for transmitting a backhaul signalfrom the plurality of groups. In this case, the plurality of RSsincluded in one group may be RSs to which the same subframe offset valueis set.

FIG. 12 shows an example in which RSs 1 to 3 which are grouped by a BShave the same subframe offset value.

Referring to FIG. 12, when the RSs 1, 2, and 3 are grouped into onegroup by the BS, the RSs 1 to 3 included in the group have the samesubframe offset value (e.g., 3). Then, the BS can transmit a backhaulsignal to all of the three RSs by using FDM in subframes 3, 6, 7, 8, 1,and 2 of each RS. However, the BS cannot transmit the backhaul signal insubframes 0, 4, 5, and 9 of each RS belonging to this group.

When an RS group created by the BS is formed in a plural number, each RSgroup may be configured to have a different subframe offset value. Ifall RS groups have the same subframe offset value, a subframe in whichno RS can receive the backhaul signal is present as described above.Since the BS does not configure the subframe as an MBSFN subframe whentransmitting the backhaul signal, the backhaul signal can be transmittedin all subframes in principle. However, if the RS is unable to receivethe backhaul signal, it causes waste of radio resources. To preventthis, a plurality of RS groups are configured to have different subframeoffset values. Then, even if the backhaul signal cannot be transmittedto any one of RS groups, the backhaul signal can be transmitted toanother RS group, and thus a radio resource can be effectively used.

However, if the BS transmits the backhaul signal in its subframes 0, 4,5, and 9, the backhaul signal is transmitted in a region other than acontrol region for transmitting a shared channel (SCH), a broadcastchannel (BCH), a paging message, or the like of the BS.

Now, a case where a BS transmits a backhaul signal to two or more RSs byusing different antennas (i.e., spatial resources), that is, a spatialdivision multiplexing (SDM) scheme, will be described. The BS transmitsthe backhaul signal to a plurality of RSs by allocating the samefrequency band within the same subframe while using different antennas.When the backhaul signal is transmitted through different antennas, thebackhaul signal can be transmitted by being processed with beamformingor a pre-coding matrix.

FIG. 13 shows resource allocation of each subframe when using an SDMscheme.

Referring to FIG. 13, a BS transmits backhaul signals to an RS 1, an RS2, and an RS 3 in a subframe n while using all frequency bands of thesubframe n. That is, the backhaul signals transmitted to different RSsoverlap in allocation of time and frequency resources, and use differentspatial resources. The BS transmits backhaul signals to the RS 2, the RS3, and an RS 4 in a subframe m. Spatial resources can be used by the useof a multi-antenna transmission and reception method. Although it isshown in FIG. 13 that the backhaul signals transmitted to different RSsfully overlap in terms of time and frequency resources, this is forexemplary purposes only, and thus the present invention is alsoapplicable to a case where the backhaul signals partially overlap.

The SDM scheme is effective when a channel between a BS and an RS have alow rank. If the channel between the BS and the RS has a low rank,multi-stream transmission from the BS to one RS by using multipleantennas becomes difficult. Therefore, utilization of a radio resourcecan be increased when the backhaul signals are concurrently transmittedto different RSs by using the multi-antenna transmission method.

The SDM scheme is similar to the FDM in a sense that a backhaul signalis transmitted to a plurality of RSs in one subframe. Therefore,configuring of a subframe offset value, grouping of RSs, or the likedescribed in the FDM scheme can also be applied to the SDM.

Next, a case where a BS transmits a backhaul signal to two or more RSsby using different time resources, that is, a time division multiplexing(TDM) scheme, will be described. The BS can transmit the backhaul signalto a plurality of RSs by allocating the same frequency band in differentsubframes.

FIG. 14 shows radio resource allocation when a backhaul signal istransmitted by using a TDM scheme.

Referring to FIG. 14, a BS transmits a backhaul signal to an RS 1 byusing all available frequency bands in a subframe n. Then, the BStransmits a backhaul signal to an RS 2 by using all available frequencybands in a subframe m (herein, n and m are integers, where n<m). Thatis, the BS transmits the backhaul signal transmitted to each of two ormore RSs in different subframes by using all frequency bands of thesubframes. In other words, the backhaul signal can be transmitted to oneRS in one subframe while using all radio resources, for example, allfrequency bands. However, if there is no need to allocate all radioresources of a subframe when the backhaul signal is transmitted to acorresponding RS, a resource of the subframe can be shared with a UEdirectly connected to the BS or with a third RS. In this case, a signaltransmitted to the UE directly connected to the BS or transmitted to thethird RS can be identified by using FDM or SDM.

When the backhaul signal is transmitted by using the TDM scheme, the BScan set a different subframe offset value of each RS. For example,referring to FIG. 11, since the RSs 1, 2, and 3 have different subframeoffset values, the BS can transmit the backhaul signal to any one of theRSs 1, 2, and 3. In this sense, the TDM scheme has an advantage inoverall radio resource utilization of a radio communication system incomparison with the FDM/SDM scheme.

FIG. 15 shows an example in which a radio resource is wasted when abackhaul signal is transmitted to a plurality of RSs by using an FDMscheme.

Referring to FIG. 15, a BS transmits backhaul signals to RSs 1, 2, and 3by using frequency bands 1, 2, and 3, respectively, in a subframe n. Thebackhaul signal transmitted at the frequency band 1 is informationeffective to the RS 1. However, the RS 1 receives the backhaul signaltransmitted to other RSs even at the frequency bands 2 and 3. Therefore,the RS 1 cannot transmit an access signal to an ReUE at the frequencybands 2 and 3 due to self interference. Likewise, the RS 2 cannottransmit the access signal at the frequency bands 1 and 3. As a result,a radio resource is wasted as a whole in a radio communication system.

When using the TDM scheme, a smaller number of MBSFN subframes allocatedto the backhaul link can be set by the RS in comparison with theFDM/SDM, and the RS can allocate more subframes to the access link. Thatis, in this method, more frequency and spatial resources are used whileusing less time resources, and thus the aforementioned waste of radioresources can be prevented.

When the backhaul signal is transmitted by the BS to a plurality of RSaccording to the FDM or SDM scheme, the RS can improve reception qualityof the backhaul signal of the radio communication system byretransmitting a backhaul signal to be transmitted to another RS afterreceiving the backhaul signal in addition to the backhaul signal to betransmitted to the RS.

FIG. 16 shows a radio communication system for performing cooperativeretransmission among a plurality of RSs.

Referring to FIG. 16, a wireless link can be established between a BSand an RS 1 or between the BS and an RS 2. A wired link or a wirelesslink can be established between the RS 1 and the RS 2. The BS cantransmit a backhaul signal to the RS 1 and the RS 2 by using differentfrequency bands f1 and f2 in the same subframe (step S1). If a backhaulsignal transmitted to the RS 1 is denoted by a backhaul signal 1 and abackhaul signal transmitted to the RS 2 is denoted by a backhaul signal2, for example, the RS 1 receives not only the backhaul signal 1 butalso the backhaul signal 2. The RS 1 can deliver the backhaul signal 2to the RS 2 either directly or after performing decoding (step S2). Thismethod is effective in particular when the RS 1 and the RS 2 areconnected in a wired fashion. The RS 2 can increase reliability of thebackhaul link and improve reception quality of the backhaul signal byreceiving the backhaul signal 2 from at least one of the BS or the RS

Alternatively, after receiving both the backhaul signals 1 and 2, the RS1 can transmit the backhaul signal 2 together when the RS 2 relays thebackhaul signal 2 to a UE 2 connected to the RS 2. In this method,signal reception quality of a UE located particularly at an edge portionof the coverage of the RS 1 and the RS 2 can be improved.

Alternatively, when an error occurs in a process of relaying thebackhaul signal 2 by the RS 2 to the UE 2 connected to the RS 2 and thusretransmission is required, the RS 1 can retransmit the backhaul signal2. In this method, diversity effect can be obtained by using cooperativeretransmission among the plurality of RSs.

Alternatively, when the RS 2 fails to receive the backhaul signal 2 andthus the BS has to retransmit the backhaul signal 2, the RS 1 canretransmit the backhaul signal 2 on behalf of the BS or in cooperationwith the BS.

When the RS has two or more sectors, the RS can configure a subframe foreach sector. When the backhaul signal is received from one sector, theRS configures a subframe as an MBSFN subframe in that sector, and alsoconfigures a subframe as an MBSFN subframe in another sector. This isbecause strong interference may occur in a sector in which the backhaulsignal is received when a signal is transmitted in another sector.

That is, the subframe is configured as the MBSFN subframe even in asector in which the backhaul signal is not received, and an accesssignal cannot be transmitted to a UE connected to the RS. In a casewhere the backhaul signal is received in any one sector in an RS havingtwo or more sectors in order to prevent waste of radio resources, it ispreferable to receive the backhaul signal from other BSs in theremaining sectors.

FIG. 17 shows an RS for establishing a backhaul link with a different BSin a different sector.

Referring to FIG. 17, when the RS receives a backhaul signal from a BS 1in a sector 1, the RS receives a backhaul signal also from a BS 2located in a sector 2. That is, when the RS receives a backhaul signalin one sector, a subframe for at least two or more sectors is configuredas an MBSFN subframe, and the backhaul signal is received from at leasttwo or more BSs.

FIG. 18 is a block diagram showing a BS according to an embodiment ofthe present invention.

Referring to FIG. 18, a BS 100 includes a processor 110 and a radiofrequency (RF) unit 120. The RF unit 120 transmits and receives a radiosignal. The processor 110 is coupled to the RF unit 120. Whencommunicating with an RS, the processor 110 selects a plurality of RSsfor transmitting a backhaul signal, and allocates different radioresources to transmit the backhaul signal to each of the selected RSs.When communicating with a UE, the processor 110 transmits informationindicating a usage of an MBSFN subframe to the UE. Although a structureof the BS is exemplified therein, the same is also applicable to the RS.

A UE 200 includes a processor 210 and an RF unit 220. The RF unit 220transmits and receives a radio signal. The processor 210 is coupled tothe RF unit 220. The processor 210 receives a usage indicator from theBS or the RS to determine whether a subframe is a T-MBSFN subframe or anF-MBSFN subframe, and receives a detailed usage indicator to determine adetailed usage of the F-MBSFN subframe. In addition, the processor 210performs a decoding process corresponding to the detailed usage.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

The invention claimed is:
 1. A method for communicating with a basestation (BS) which uses a BS frame in a wireless communication system,the method performed by a relay station (RS) and comprising:determining, by the RS based on both a timing applied to the BS frameand a timing applied to a RS frame, at least one subframe of the RSframe for receiving a signal from the BS; and receiving, by the RS, thesignal in the at least one subframe of the RS frame, wherein the atleast one subframe of the RS frame is configured as a MulticastBroadcast Single Frequency Network (MBSFN) subframe by the RS, whereinthe at least one subframe is determined according to the timing appliedto the BS frame with an exception of any subframes of the BS frame thatare not configurable as an MBSFN subframe according to the timingapplied to the RS frame, and wherein subframes of the RS frame that arenot configurable as a MBSFN subframe according to the timing applied tothe RS frame are fixed.
 2. The method of claim 1, wherein a frequencyband at which the RS receives a signal from the BS is identical to afrequency band at which the RS transmits a signal to a user equipmentconnected to the RS.
 3. The method of claim 1, further comprising:receiving a configuration message indicating the at least one subframefrom the BS by using a higher layer signal.
 4. The method of claim 1,wherein if the RS frame comprises 10 subframes and the 10 subframes areindexed from 0 to 9, the subframes that are not configurable as a MBSFNsubframe according to the timing applied to the RS frame correspond tosubframe 0, 4, 5, and 9 of the RS frame.
 5. A relay station (RS)configured to communicate with a base station (BS) which uses a BS framein a wireless communication system, the RS comprising: a radio frequency(RF) unit; and a processor operatively connected to the RF unit andconfigured to: determine, by the RS based on both a timing applied tothe BS frame and a timing applied to a RS frame, at least one subframeof the RS frame for receiving a signal from the BS and receive, by theRS, the signal in the at least one subframe of the RS frame, wherein theat least one subframe of the RS frame is configured as a MulticastBroadcast Single Frequency Network (MBSFN) subframe by the RS, whereinthe at least one subframe is determined according to the timing appliedto the BS frame with an exception of any subframes of the BS frame thatare not configurable as an MBSFN subframe according to the timingapplied to the RS frame, and wherein subframes of the RS frame that arenot configurable as a MBSFN subframe according to the timing applied tothe RS frame are fixed.
 6. The RS of claim 5, wherein a frequency bandat which the RS receives a signal from the BS is identical to afrequency band at which the RS transmits a signal to a user equipmentconnected to the RS.
 7. The RS of claim 5, the processor receives aconfiguration message indicating the at least one subframe from the BSby using a higher layer signal.
 8. The RS of claim 5, wherein if the RSframe comprises 10 subframes and the 10 subframes are indexed from 0 to9, the subframes that are not configurable as a MBSFN subframe accordingto the timing applied to the RS frame correspond to subframe 0, 4, 5,and 9 of the RS frame.